1.Field of the Invention
The present invention relates to a broadband wireless access system, such as WiBro (Wireless Broadband), and more particularly to a data transmission/reception method for compensating for a packet loss occurring in a wireless channel in an environment, which provides a multicast & broadband service (hereinafter, referred to as MBS) based on a broadband wireless access system.
2.Description of the Related Art
A broadband wireless access system (e.g. Wibro) refers to an IP (Internet Protocol)-based wireless data transmission system using OFDMA/TDD (Orthogonal Frequency Division Multiple Access/Time Division Duplex)-type wireless transmission technology, which features upward/downward asymmetric transmission. In addition to a voice service, the broadband wireless access system supports a combined multimedia application service, including various low/high-speed data services, high-quality moving images, etc. Such a broadband wireless access system enables access to PSTN (Public Switched Telephone Network), PSDN (Public Switched Data Network), IMT-2000 (International Mobile Telecommunication-2000) network, Internet, or ATM (Asynchronous Transfer Mode) network in a mobile or stationary environment based on a wireless medium using a broadband (e.g. 2 GHz, 5 GHz, 26 GHz, or 60 GHz), and supports a channel transmission rate of 2 Mbps or higher.
The broadband wireless access system provides not only voice and packet data communication services, which have been available from conventional communication systems, but also high-speed multimedia communication services, such as an MBS, to users. Particularly, when a communication system provides an MBS, its transmitter, e.g. RAS (Radio Access Station), transmits (i.e. multicasts) the same data (i.e. broadcasting contents) to a number of receivers, e.g. MSs (Mobile Stations).
The frame structure of a conventional Wibro system will now be described with reference to FIG. 1. In a Wibro system, a frame must be processed within 5 ms and is temporally divided into a DL (Downlink), a TTG (Transmit/receive Transition Gap), a UL (Uplink), and an RTG (Receive/transmit Transition Gap). A portion of the frame is allocated to the DL, and another portion is allocated to the UL. The TTG and RTG between the DL and the UL provide the system with time for transition into the CL and DL. In FIG. 1, the vertical axis of the frame refers to an OFDM subcarrier, and the horizontal axis refers to an OFDM symbol.
The multicasting zone shown in FIG. 1 refers to a portion of the DL allocated for multicasting. Particularly, the multicasting zone is used to transmit broadcasting data and provides an MBS. The DL_MAP (Downlink_MAP) shown in FIG. 1 contains information regarding which data is transmitted via which zone inside the DL. In addition, the DL_MAP contains information regarding the position of the multicasting zone in the Wibro frame. The MBS_MAP (Multicast Broadcast Service_MAP) shown in FIG. 1 is positioned within the multicasting zone and contains information regarding the transmission cycle of MBS data, the data coding mode, and the physical layer for data transmission.
FIG. 2 shows an exemplary transmission scheme of frames from an RAS to an MS in a conventional Wibro system. Particularly, FIG. 2 shows exemplary cycles of the transmission of broadcasts A, B, and C via frames in a Wibro system. It is clear from FIG. 2 that broadcast A is transmitted every frame; broadcast B is transmitted every second frame; and frame C is transmitted every third frame. Such transmission cycles are described in MBS_MAP shown in FIG. 1. Data is encapsulated in an IP packet for each broadcast and transmitted. Each IP packet is transmitted via a frame.
Considering the current wireless communication technology, the low reliability of wireless channels makes any control regarding packet losses impossible. In the case of normal data, which has no real-time properties, the receiver may request the retransmission of a lost packet. However, in the case of a broadcast having real-time properties, retransmission is possible only when the roundtrip time is short and the receiving buffer has a large size. In addition, due to the nature of an MBS service, i.e. because a transmitter transmits the same data to a number of receivers, it is impractical to retransmit a lost packet of a specific user. Further, a packet loss occurring in a wireless channel directly degrades the screen quality, which needs to be compensated for.
Current wireless communication services show drastic differences between a daytime period with many users and a nighttime period with few users. Therefore, a wireless communication service provided by Qualcomm Inc., “MediaFLO,” has introduced a service named “ClipCasting” in an attempt to efficiently use the bands, which are not used during the nighttime period. According to this service “ClipCasting,” a user can designate desired moving image clips in advance, and the designated contents are automatically downloaded to the user's terminal during an idle period, e.g. during the nighttime period, so that the user can watch the stored contents. This service enables users an efficient use of wireless bands with little cost at night. However, such a service has a problem in that, even when contents are downloaded to the terminal at night to be watched at a later time, packet losses may still exist in wireless channel environments. When a multicasting service (e.g. broadcast) is downloaded at night, retransmitting of the lost packets is not easy. It is far more difficult for a receiver to automatically secure a separate unicasting channel and receive the lost packets only. As a result, packet losses occurring in wireless channels degrade the screen quality.